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Re: Concrete vs. Steel

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> 1.  To a point as with any material, concrete works fine in bending.  But,
> in cyclic shear + tension it fractures one side at a time until all that is
> left is two cone shaped ends with rebar holding them together and no
> frictional surface.  Steel is ductile and concrete is brittle.

The whole point of putting reinforcement in concrete is to create a ductile
failure mechanism.  This is part of the reason the code established the 75% rho
balanced limit and the restrictive shear and tie requirements.  Because there
is some cracking involved, energy dissipation is favorable with concrete.

> 2.  The bay bridge failed in shear (I SAW the joints) at d/2 to d below the
> knee.

I think perhaps you are referring to the double deck highway that collapsed?  I
agree with Mark Swingle, the Bay Bridge is steel as far as I can tell.

> 3.  You can't fix wrongly detailed concrete.

Up to a point, you can, if you look at some of the carbon fiber wrap technology
that is out there.  You also have the option of adding shear walls, or
supplementary framing.  These two options are common.  This is largely a
non-issue, since steel building are often uneconomical to repair when
fire-proofing removal and replacement is taken into consideration, so you are
often better off adding supplementary structure in either case.

> 4.  In the event of collapse, with concrete people get buried and steel
> maintains a space (better)(~).

I am not sure where this has been established.  Nice voids have been known to
form between concrete double tees, and between concrete girders.  In either
case, if you're in a large enough building, it's probably all over if it comes
down.  Don't kid yourself about the extent to which a concrete filled metal
deck in a steel building is going to crumble if the frame fails or how heavy a
deck is going to be if it lands on you.

> 5.  With n=9(+) and weight per unit vol. =3.3 for steel/concrete the
> strength to weight ratio makes steel a better material where mass is a
> critical factor.

When you start considering the concrete filled metal deck flooring that you end
up adding into a steel building, and all of the extra beams you need to support
the typically shorter spans in a steel building, the floor system may only end
up being 1/3 or so lighter.  This is a helpful reduction, but it is not a
dramatic night versus day change in the design approach.  If you could use a
metal deck alone, without concrete fill, the weight of the steel floor system
would be much more favorable.  Unfortunately, this flooring would be bouncy and
noisy, and so much of the benefit of steel's weight to strength ratio is lost
here.  Developing adequate shear in the diaphragm and some degree of monolithic
behavior in the system would get pretty difficult, anyway, so maybe it's no
great loss.

Paul Crocker.